1. Field of the Invention
The present invention relates to a planar lightwave circuit type variable optical attenuator that utilizes a Mach-Zehnder interferometer and adjusts an optical length with a thin-film heater.
2. Description of the Related Art
In Wavelength Division Multiplexing (WDM) optical communication, optical signals emitted from a plurality of light sources and having different wavelengths in the band of 1.55 xcexcm are transmitted through a single optical fiber transmission line and received by light receivers allocated for each wavelength, thereby realizing large-capacity optical communication. Respective powers of the optical signals at the light receivers are required to be equal to one another. A variable optical attenuator is necessary to meet that requirement.
As variable optical attenuators, there are known a planar lightwave circuit type utilizing a Mach-Zehnder interferometer and adjusting an optical length with a thin-film heater, a bulk type optical system in which an optical filter is driven by a stepping motor, and a bulk type optical system utilizing a Faraday rotator. Of those variable optical attenuators, the planar lightwave circuit type has many advantages over the other types: it can be downsized through integration and is superior in productivity for mass production, and it has smaller loss.
FIG. 1 shows a conventional planar lightwave circuit type variable optical attenuator 1. The illustrated variable optical attenuator 1 includes a Mach-Zehnder interferometer 101 comprising two optical waveguide arms 10, 20 buried in a clad 70 formed on a substrate 100, and two directional couplers 50, 51 for coupling the optical waveguide arms 10, 20 together at their both end. A thin-film heater 110 is disposed, extending along the optical waveguide arm 10, on the surface of the clad 70. In FIG. 1, xe2x80x9cIinxe2x80x9d represents an input optical power, and xe2x80x9cIoutxe2x80x9d represents an output optical power.
An electric power W supplied to the thin-film heater 110 is controlled by a control unit 200 for adjusting the temperature of the thin-film heater 110. Upon heating of the thin-film heater 110, the temperature of the optical waveguide arm 10 rises and the refractive index of the optical waveguide arm 10 changes with the thermooptic effect. As a result, a difference in optical length between the optical waveguide arms 10 and 20 is also changed, whereby the output optical power Iout can be varied according to the optical interference.
A difference xcex94L0 between the arm lengths under a condition in which no electric power is supplied to the thin-film heater 110, is set to zero or xcex0/2neff (xcex0 is the central wavelength of an operating wavelength band and neff is the effective refractive index of the optical waveguide) corresponding to a phase difference 180xc2x0. When the arm length difference xcex94L0 is zero, the input optical power is output to an output port positioned in a diagonal relation to an input port. When the arm length difference xcex94L0 is xcex0/2neff, the input optical power is output to the output port positioned on the same side as the input port.
When the thin-film heater is disposed on the clad, the optical waveguide arm is susceptible to thermal stress in the vertical direction because of a difference in thermal expansion coefficient between the core and the clad. The thermal stress generates double refraction in the optical waveguide arm because of photoelasticity and hence increases polarization dependence of optical characteristics. In the optical attenuator of FIG. 1, for example, as the heating temperature of the thin-film heater 110 rises, a larger attenuation is obtained, but a Polarization Dependent Loss (PDL) is also increased at the same time.
With respect to such problem, a reference xe2x80x9cProceedings of the 2001 Institute of Electronics, Information and Communication Engineers General Conference, C-3-64, p. 229, Lower PDL of PLC Type Variable Attenuatorxe2x80x9d describes a technique for reducing the PDL by means of thermal stress releasing grooves formed on both sides of an optical waveguide arm.
It is an object of the present invention to provide a planar lightwave circuit type variable optical attenuator in which a small PDL is maintained even when attenuation is large.
To achieve the above object, the present invention provides a planar lightwave circuit type variable optical attenuator including a Mach-Zehnder interferometer formed on a substrate. The Mach-Zehnder interferometer comprises two optical waveguide arms formed on the substrate, and thin-film heaters formed respectively over the two optical waveguide arms, the two optical waveguide arms having lengths not equal to each other.
Assuming that an effective refractive index of the optical waveguide arms is neff and a central wavelength of an operating wavelength band is xcex0, a difference xcex94L0 between the lengths of the two optical waveguide arms may satisfy the following relationship:
0.36xc3x97xcex0/neffxe2x89xa6xcex94L0xe2x89xa60.47xc3x97xcex0/neff
An attenuation may be in the range of 7 to 21 dB when no electric powers are supplied to the thin-film heaters for adjusting the optical lengths of the optical waveguide arms. Plural sets of Mach-Zehnder interferometers may be formed on the substrate in parallel.
The present invention is further explained below by referring to the accompanying drawings. The drawings are provided solely for the purpose of illustration and are not intended to limit the scope of the invention.